The invention relates to a process for filtering cellulose acetate solutions.
Cellulose acetate in organic solvents is used to produce acetate rayon, foils and binders. The purification of the acetate solution takes place by means of filtration; advantageously a settling filter is used.
The use of such a filter is disclosed in Swiss Pat. No. 446274. Pulverized and very fine grained inorganic matter having a particle size within the range of 50-150 microns are used as filter aids in a filter of this type, according to the disclosure of Swiss Pat. No. 477 215.
In the case of spinnable acetic acid esters of cellulose (from which the so-called cellulose acetate precipitates), there is problem of purifying the filtrate in such a manner as to avoid clogging the spinning nozzles. Up to now, all of the conventional processes have operated only partially satisfactorily, since the expensive spinning nozzles must be replaced too frequently. One of the most important values for determining the characteristics of the filtration behavior of viscous solutions (such as acetylated cellulose) is the filter clogging value, known as the "FZ value" (Verein der Zellstoff- und Papier-Chemiker und -Ingenieure, Merkblatt (pamphlet) III/6, B/68 of Oct. 1, 1968).
The formula for determining the FZ value is: ##EQU1## WHERE T.sub.1 AND T.sub.2 ARE TWO DIFFERENT FILTERING TIMES IN MINUTES, BOTH BEING CALCULATED FROM THE BEGINNING OF THE ENGAGEMENT OF THE FILTER AND THE SOLUTION TO BE FILTERED; AND M.sub.1 AND M.sub.2 ARE THE QUANTITIES OF FILTRATE IN GRAMS PASSING THROUGH THE FILTER RESPECTIVELY OVER THE TIMES T.sub.1 AND T.sub.2. It is assumed that the time t.sub.1 is chosen to be sufficiently long after commencement of filtering so that significant filtration can occur.
Tests with quartz sand have led at best to FZ values between 70 and 100 per gram. However, as a rule these FZ values are higher, and the higher the FZ value the more rapidly the pores of a filter used filtering of viscous solutions will become clogged.